The invention relates to data display terminals, in particular video terminals for the visual display of information on a cathode ray tube. Video terminals of the type to which this invention relates may be used by a person to communicate, for example with a computer located at some distance via data communication lines, for example commercial telephone lines. The invention relates particularly to a video terminal which employs as its display device a television receiver.
Known data terminals usually include a console with some type of cathode ray screen, a keyboard for writing various messages, commands and other control information and may have an acoustical coupler which permits the transfer of information to and from the data terminal via commercial telephone lines. The acoustic coupler normally accepts the handset portion of a standard telephone set permitting the acoustical exchange of signals with the telephone set. The type of data terminal referred to above has a large number of applications, among which are the interrogation of an interaction with data banks, the transmission and exchange of communication, the dialogue with a computer, process control and numerous others. Generally, it is desirable if these data terminals are easily transported but they may also be permanently installed.
While there appears to be a universal utility for these data terminals, their general use in public is still limited because of the relatively high cost of acquisition. In order to lower the overall cost of this type of equipment, it has been proposed to employ as the visual display device a commercial television receiver for displaying written text and graphical symbols. Ordinary television receivers are produced in vast numbers and are generally accessible to any potential user of data terminals.
Inasmuch as the majority of potential users thus has access or possession of a television set and normally can make use of a standard telephone set and because the keyboard itself is not a very high-priced item, it would be possible to considerably lower the cost of a data terminal if an electronic processor were designed which would appropriately combine the functions of these various elements to produce a data video terminal which has high flexibility for handling various types of data. Two principal concepts stand out for use in the general area of performing the required processing tasks on a data terminal. The first of these makes use of universal microprocessors, generally known under the acronyms M.P.U., C.P.U., etc. Such microprocessors may be coupled with standard components and can be programmed, thereby giving the system in which they are used a great deal of flexibility but incurring a substantial cost. The second concept aims at providing a specialized processor for a particular application. Such a processor is constructed by judicious combination of circuit elements which may include those of medium scale integration (M.S.I.) so that the overall flexibility and performance of the processor is limited but the attendant cost is reduced. The present invention differs from these two concepts in that it describes a method and means for producing a specialized processor of high flexibility and capability associated with a limited number of standard M.S.I. components and capable of performing the required functions to permit a very flexible system for communication by means of a video screen. Furthermore, the circuitry according to the invention is capable of being included in a large scale integration processor in a single integrated circuit block.